CD-ROM (compact disk read-only-memory) media have achieved widespread acceptance in recent years as a very high density storage medium for use with personal computers. Formatted storage capacities of about 550 Mbytes on a 12 cm diskette are routinely achieved. A common CD-ROM diskette is very similar in appearance and construction to a compact digital audio disc, and comprises a plastic substrate having a circular or spiral pattern of grooves, more specifically, a series of pits surrounded by land areas, replicated on a surface and in which digitized data or information is conveyed by alterations in the grooved pattern. Such media are typically prepared by first making a master disc via optically scanning a substrate coated with a photo resist and then removing nonhardened resist to produce the desired grooved pattern. Such masters, or second generation stampers made from the masters are then used in production stamping equipment to replicate the grooves onto plastic substrates.
In particular, such master discs may be coated with a metallic layer, such as copper, in which the grooves are formed by well-known photo resist techniques, while the second generation stampers are formed by nickel-plating and molding operations. Upon producing the final replicated surface in a production disc, that surface may be overcoated with a reflective layer, i.e., a thin-film of metal, aluminum or the like, that layer in turn overcoated with a protective layer, and a printable layer placed thereover. During playback, a laser is focused through the transparent rear surface onto the reflective layer, and reflected light modulated by the grooved pattern is detected. Small scratches and dust on the light incident surface have little effect on signal quality as they are out of focus.
The spiral tracks on such discs are generally played from the center out while initially rotating at about 500 rpm and gradually slowing to about 200 rpm, maintaining a constant linear velocity of about 1.25 m/sec. Following detection, the playback signal is error corrected and ultimately fed to appropriate converters, computers and playback/display devices.
As no physical contact with the tracks is allowed, tracking of the laser beam is provided by optical servoing techniques. Track densities exceeding 15,000 tracks per inch are routinely achieved. Servo information along with user specific data is typically provided via the differing reflectivities of the lands and pits, respectively, making up the spiral pattern. The output from an optical sensor is separated into user data and servo data components, and the latter is coupled in a feed-back loop to sensor positioning devices which maintain the sensor centered on the respective track.
Unlike competitive storage media such as magnetic tapes and discs, solid state memories, etc., all of which can and are routinely erased and re-recorded to provide additional and/or updated information, CD-ROM media discussed above are limited in that the information provided at the time of manufacture cannot be altered to add additional or update information. Rather, entirely new masters and stampers must be produced in order to provide discs containing desired new information.